The technical field of this invention is that of nondestructive materials characterization, particularly quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using magnetic field based or eddy-current sensors. Characterization of bulk material condition includes (1) measurement of changes in material state, i.e., degradation/damage caused by fatigue damage, creep damage, thermal exposure, or plastic deformation; (2) assessment of residual stresses and applied loads; and (3) assessment of processing-related conditions, for example from aggressive grinding, shot peening, roll burnishing, thermal-spray coating, welding or heat treatment. It also includes measurements characterizing material, such as alloy type, and material states, such as porosity and temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness, displacement or changes in relative position, coating thickness, temperature and coating condition. Each of these includes detection of electromagnetic property changes associated with either microstructural and/or compositional changes, or electronic structure (e.g., Fermi surface) or magnetic structure (e.g., domain orientation) changes, or with single or multiple cracks.
A specific application of these techniques is the inspection of engine blades for cracks in the dovetail regions. This is an area with significant curvature and often has other geometric features that help to hold the blade in an engine disk slot. This curvature and the geometric features can limit the effectiveness of the conventional inspection techniques, such as eddy current and ultrasonic techniques, for the detection of cracks.
Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field at the same frequency, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks.
For engine disk slot inspection, differential coil designs are typically used. These designs sense local changes in the flow of eddy currents by comparing signals in neighboring regions. For clusters of cracks, this “comparison” could occur between a sensing region on a large crack and one on a neighboring small crack or cluster of small cracks. This could significantly alter (reduce) the differential signal. Furthermore, differential coil designs are affected by local changes in proximity between the two sensed regions, e.g., if one region of a differential coil is at a different lift-off than the other.